Electromagnetic levitation casting apparatus having improved levitation coil assembly

ABSTRACT

An electromagnetic levitation casting apparatus having an improved levitation coil assembly employing flux concentration devices, is provided. The improved levitation coil assembly comprises a plurality of slotted annular slugs surrounding a tubular casting vessel within which a liquid metal column is to be levitated and solidified by cooling pursuant to the General Electric Levitation Casting (GELEC (®)) process. Each of the slotted annular slugs is inductively coupled to a respective electromagnetic field producing coil having a large number of turns surrounding the slug. Each slotted annular slug serves to concentrate the magnetic field produced by its associated primary mutli-turn coil to substantially the interior cross sectional area of the tubular casting vessel it surrounds and functions as a current step-up transformer. A separate slotted annular slug and associated surrounding electromagnetic multi-turn field producing coil is provided for each phase winding of the multi-phase excitation employed in the improved levitation coil assembly.

TECHNICAL FIELD

This invention relates to an improved apparatus for the casting ofcontinuous metal rods.

More specifically, the invention relates to an electromagneticlevitation casting apparatus having an improved levitation coil assemblyfor the continuous casting of metals in long lengths using anelectromagnetic levitation casting process described in U.S. Pat. No.4,414,285--issued Nov. 8, 1983 for "Continuous Metal Casting Method,Apparatus and Product"--Hugh R. Lowry and Robert T. Frost, inventors,and assigned to the General Electric Company.

BACKGROUND PRIOR ART PROBLEM

The above-referenced U.S. patent discloses and claims a unique processfor the continuous casting of metal rod in the presence of a levitatingelectromagnetic field which is used to overcome frictional, adhesive andgravitational forces normally acting on the cast rod as it solidifiesfrom the molten state. For this purpose, a multiturn coil connected to apolyphase source of electrical energy is employed to provide thelevitating electromagnetic field that acts on a molten metal columncontained within a tubular heat exchanger/casting vessel as itsolidifies. The levitation electromagnetic field is in the form of anupwardly travelling electromagnetic field that both constrains themolten metal column and maintains it in a substantially weightlesscondition with reduced hydrostatic head in the solidification region ofthe heat exchanger/casting vessel whereby the solidified rod product canbe continuously withdrawn by a rod removal mechanism acting on thesolidified rod product after it has passed through the heatexchanger/casting vessel.

The construction and operation of an electromagnetic levitation castingapparatus having the above-described capabilities and designed foroperation at the high power levels required and in the high temperatureenvironment encountered, presents several problems.

The General Electric levitation casting process (hereinafter referred toas the GELEC (TM) process) requires a strong, upward travellingelectromagnetic field to be created in the interior of the tubularcasting vessel/heat exchanger assembly which supports and contains theliquid metal column while it is solidifying. In the type of levitatingapparatus built to date for practicing this process, the levitatingfield is generated by currents in the range of 500-1,000 amperes flowingin a 36-turn levitation coil. Since reasonably sized insulated wirescannot carry such currents continuously, water cooled copper tubingcurrently is used for the levitation coil. This coil is placed in closeproximity to the exterior wall of the heat exchanger which in turnsurrounds a tubular casting vessel made of refractory material. Such acoil maximizes the magnetic field intensity within the interior of thetubular casting vessel.

The necessity to provide for an adequate cooling water flow through thecopper tubing forming the coil while also making electrical connectionsto the tubing from the cables or bus bars that carry the heavyenergizing currents, presents many problems from a mechanical andelectrical engineering standpoint. Additionally, since a magneticlevitation coil made from copper tubing must consist of only arelatively low number of turns (typically 3 turns per phase), theresulting levitating magnetic field is not completely uniform. Thisnon-uniformity is believed to produce slight non-homogenity in grainstructure occassionally observed in the cast rod produced by theprocess.

A practical solid state generator of high frequency polyphase power inthe range of 10-50 kilowatts has an output voltage of roughly 100-500volts. This high voltage, low current output must go through a step-downtransformer with forced air or water cooling in order to produce the lowvoltage, high current required to energize the present levitation coildesign described briefly above. A high frequency 10-50 kilowatt, threephase step-down transformer (or three single phase transformers) isexpensive, large and somewhat difficult to design and fabricate.Further, the step-down transformer and associated high current supplycables or bus bars feeding the levitation coil assembly used to datehave not been entirely satisfactory and a simpler, less expensive designis very desirable.

To overcome these difficulties with an efficient and economicalstructure, the present invention was devised

The present invention provides a unique and non-obvious solution to theabove-discussed problems through the use of an improved levitation coilassembly that makes use of a novel arrangement of flux concentrationdevices. While the use of flux concentration devices in the productionof large magnetic fields in order to improve the coil life of multi-turncoils used to produce the large magnetic fields has been described inthe prior art, it has not been used or suggested for use heretofore withrespect to the electromagnetic levitation of molten metals. One priorart description of a flux concentrator appears in an article entitled"Flux Concentrator For High Intensity Pulsed Magnetic Field" by Y. B.Kim and E. D. Platner in the Review of ScientificInstruments--7/59--pages 524-533. A different form of flux concentrationdevice for use in eddy-current testing apparatus is disclosed in U.S.Pat. No. 3,872,379--issued Mar. 18, 1975 for "Eddy Current TestingApparatus Using Slotted Mono-Turn Conductive Members"--John P. Wallaceand Robert A Brooks--inventors.

In practicing the present invention a unique and non-obvious levitationcoil assembly employing flux concentration devices for use in theelectromagnetic levitation of molten metal is provided and comprises aplurality of slotted annular slugs surrounding a tubular casting vesselwithin which a liquid metal column is to be levitated and solidified bycooling pursuant to the GELEC (TM) process. Each of the slotted annularslugs is inductively coupled to an electromagnetic field producing coilhaving a large number of turns surrounding the slug. Each slottedannular slug serves to concentrate the magnetic field produced by thecoil to substantially the interior cross sectional area of the tubularcasting vessel it surrounds, and functions as a current step-uptransformer. A separate slotted annular slug (or stack of thin slugs)and associated surrounding electromagnetic field producing coil isprovided for each phase of the multi-phase excitation provided for theGELEC (TM) levitation coil assembly.

The use of the slotted annular slug magnetic flux concentrator devicesprovides a number of important and non-obvious advantages. One advantageis that it will minimize or eliminate coil induced field variationscaused by inevitable variations in the construction of multi-turn liquidcooled copper tube coils used heretofore. A further advantage is thatthe slotted annular slug members will be uniformly closer to the castmetal column and will increase the electromechanical restoring force onthe column thereby allowing better control of the cast metal columndiameter. Additionally, and equally important is that the improvedlevitator assembly using the slotted annular slug flux concentratorsallows a higher levitating magnetic field to be generated with a lowerimpedance device. This in turn reduces the voltage requirements for thelevitator coil assembly and the total power requirements and providesgreater electrical efficiency. For example, a 40% decrease in impedancewould reduce both the required driving voltage and input power by 40%.Due to the addition of the flux concentrator device the flux density atthe inside periphery of the energizing coil is effectively transferredto, and reproduced at, the inner periphery of the central opening in theflux concentrator disk. This displacement of the flux from theenergizing coil periphery to the disk inner periphery of the centralopening in the disk (i.e., the surface closest to the levitated metal)is precisely what is desired in the GELEC (TM) process. Lastly, the fluxdensity at the center of the coil is higher with the flux concentratordisk in place than without it, which is desirable. Even more important,however, is that the gradient (i.e., change in flux density withdistance) of the flux from the center point outward is also much higherwith the flux concentrator disk in place. The gradient of the flux iswhat determines the inward containment pressure on the levitated metalcolumn so a higher value of gradient is desirable.

In the existing GELEC (TM) apparatus, the input impedance of thelevitator coil changes considerably when the molten copper rises up intothe levitator coil/heat exchanger assembly at the start of a castingrun. This impedance change is caused by the electrical load coupled intothe levitator coil when copper (molten and solidified) exists in theinterior of the coil. A desired value of the levitator coil current setbefore the run by adjusting the inverter output voltage must thereforebe reset quickly to the desired value after the start of the run becauseof this change in coil impedance. The addition of the flux concentratordevice virtually eliminates this problem because the highly conductivedisks will have already lowered the energizing coil impedancedrastically and introduction of molten metal in close proximity to thecentral opening of the concentrator disks will have little or no furthereffect on the energizing coil impedance. The flux concentrator devicetherefore makes it possible to more accurately set an optimum levitatorcoil current before casting starts and hold this value of current duringthe critical start-up operation.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and many of the attendant advantagesof this invention will be appreciated more readily as the same becomesbetter understood by reference to the following detailed description,when considered in connection with the accompanying drawings, whereinlike parts in each of the several figures are identified by the samereference characters, and wherein:

FIG. 1 is a diagramatic sketch of a slotted, annular slug fluxconcentrator device employed in constructing an improved electromagneticlevitating coil assembly according to the invention;

FIG. 2 is a voltage versus distance characteristic curve indicative ofthe flux field of a multi-turn electromagnetic induction coil andillustrates the difference in flux concentration achieved where the coilhas only an open air center as opposed to a coil having a slottedannular slug flux concentrator such as shown in FIG. 1, insertedtherein;

FIG. 3 is a cross sectional view of one embodiment of an improvedlevitation coil assembly constructed according to the invention;

FIG. 4 is a schematic functional block diagram of an electromagneticlevitation casting apparatus having an improved levitation coil assemblyconstructed according to the invention;

FIG. 5 is a plan view of an alternative construction for a slottedannular slug flux concentrator employed in fabricating an alternativeform of the improved levitation coil assembly shown in cross section inFIG. 6; and

FIGS. 7, 7A and 7B illustrate still another embodiment of the inventionsuitable for use in casting flat plate having a rectangular crosssection.

BEST MODE OF PRACTICING THE INVENTION

FIG. 1 is a planar end view showing a slotted annular slug 11 surroundedby a multi-turn coil 12 of insulated wire which surrounds the outerperiphery of annular slug 11. The multi-turn coil 12 is excited from analternating current power source 13. A central opening 14 is formed inannular slug 11 and a non-conductive slot 15 extends from the outerperiphery of slug 11 all the way through to central opening 14. Aninstantaneous current flow through the outer primary multi-turn coil 12in the direction indicated by the arrows 16 induces an opposite currentflow around the outer periphery of annular slug 11 indicated by thearrows 17. Since the ampere-turns in slug 11 must essentially equal theampere-turns in the energizing primary coil 12, the current flow in theslug 11 (which is the equivalent of a single-turn secondary coil) willbe very large. Without a non-conductive slot as shown at 15 in the slug,the current 17 would be equally high but, because of the skin effectphenomena, would flow predominately near the outer edge of the disc andthe electromagnetic field produced within the interior central opening14 would be minimal. However, the existence of the non-conductive slot15 through the annular slug from the central opening 14 to the outerperiphery of the slug forces the current flow in the slug to travelalong the sides of the slot 15 and around the periphery of the centralopening in the manner shown by the arrows 18. By thus forcing thecurrent to flow around the periphery of the central opening 14, adesired high electromagnetic field is produced within the centralopening 14.

In essence then, the outer multi-turn primary energizing coil 12 andslotted slug arrangement 11 acts as a current step-up transformer sothat a high voltage, low current energizing coil 12 of relatively largediameter and having a large number of turns will produce a low voltage,high current flow around the central opening 14 of annular slug 11.Opening 14 is much smaller in diameter than coil 12 and hence creates ahigh magnetic flux within the central opening. It is this highlyconcentrated, central magnetic flux which is needed for practicing theGELEC (TM) process.

From the foregoing description, it will be appreciated that the fieldproducing current flow that provides the levitating electromagneticforce necessary in the GELEC (TM) process is essentially the flow aroundthe inner periphery of central opening 14 of slotted annular slug 11.This feature provides a major advantage in that the apparent innerdiameter of the energizing coil that produces the levitating field isreduced by the insertion of the slotted annular slug as if it werereplaced by a coil of the same number of turns per meter, but with theinner diameter of the slotted annular slug 11. This allows a highermagnetic field to be generated with a lower impedance device and reducesthe total power requirement of the energizing source. The annularslotted slug member 11 therefore acts as a flux concentrator andeffectively increases the flux produced by the multi-turn coil by afactor substantially equal to the total area of multi-turn coil 12 crosssection divided by the inside area of central opening 14.

The results of measurement tests run with a magnetic field sensing probeon the concentrating effects of a slotted annular slug member 11 isshown in FIG. 2 of the drawings. In FIG. 2, the curve shown in dottedline illustrates the magnetic field flux density B measured in Gaussesfrom the center of a multi-turn coil such as 12 without the presence ofa field concentrating slotted annular slug member 11. The curve shown insolid line in the upper right corner of FIG. 2 illustrates the resultsof the measurements taken after insertion of a slotted annular fieldconcentrator 11. This data was taken with respect to a multi-turnenergizing coil having ten turns wound in a planar loop having a 6.5centimeter diameter. The slotted annular slug member 11 of copper had acorresponding 6.5 cm outer diameter, a 1.5 cm diameter central openingand was 0.6 cm thick. The multi-turn coil was excited by a 1.0microsecond pulse repeated at a frequency of one kilohertz. From FIG. 2,it will be seen that the introduction of the slotted annular slug member11 results in a large integrated increase in flux within the centralopening 14.

In an improved levitation coil assembly for the GELEC (TM) apparatus,the levitating field producing current flow that will drive the metalcasting process is essentially the flow around the central opening 14 ofthe slotted annular slug member 11. This provides a major advantage overpreviously used multi-turn coil driving arrangements in that the currentflow path is not constrained so that the current flow path and resultingelectromagnetic force it produces can concentrate at those points aroundthe periphery of the central opening 14 where the molten metal columnpassing through opening 14 has a larger diameter. This in effectincreases the apparent stiffness of the containment effects of thelevitating electromagnetic fields acting as a mold and should assist indecreasing the mean variation in the cast solidified rod productdiameter.

FIG. 3 of the drawings illustrates a preferred construction for animproved levitation coil assembly for use in the GELEC (TM) apparatuswhich employs a plurality of slotted annular slug members such as shownin FIG. 1 as flux concentration devices. In FIG. 3 an elongated tubularcasting vessel is shown at 19 which is fabricated from a hightemperature refractory material such as, but not limited to, ceramics,graphite, zirconia or the like. Casting vessel 19 is cooled by anannular liquid cooled heat exchanger 21 that immediately surroundstubular casting vessel 19. As will be explained hereafter with respectto FIG. 4, means are provided for continuously supplying a liquidcoolant through the annular heat exchanger 21. Concurrently, liquidmetal shown at 23 is delivered into the lower end of tubular castingvessel 19 where it rises. As the molten metal 23 rises in the tubularcasting vessel 19 it will be levitated by the levitating electromagneticfield and cooled substantially at a molten metal-solidified metalinterface shown at 24 and thereafter can be withdrawn from the topportion of tubular casting vessel 19 as solidified rod product 25. Themanner in which the solidified rod product 25 is withdrawn also will bedescribed more fully hereafter with respect to FIG. 4 of the drawings. Asmall gap indicated at 22 is created between the exterior surfaces ofthe levitated metal column 23 and the interior surrounding surfaces ofthe tubular casting vessel 19 by the containment effect of theelectromagnetic field. When the liquid metal column solidifies it willfurther shrink in diameter thus maintaining this gap as the rod cools.

Around the solidification region indicated at 24, electromagneticlevitation field producing means are provided. This means is comprisedby a novel levitator coil flux concentrator assembly according to theinvention for producing an electromagnetic levitation field that reducesthe hydrostatic head of the liquid metal column in the solidificationregion 24 and maintains the liquid metal column in a substantiallyweightless condition within this region while simultaneously maintaininga predetermined dimensional relationship between the outer surface ofthe liquid metal column and the interior surrounding surfaces of thecasting vessel 19 by the containment affect of the levitatingelectromagnetic field. For this purpose, means (to be describedhereafter with relation to FIG. 4) are provided for establishing andmaintaining the value of the electromagnetic field so that the crosssectional dimension of the liquid metal colum 23 is sufficiently largeto preclude formation of a substantial gap that would introduce highthermal losses between the outer surfaces of the liquid metal column andthe interior surrounding surfaces of the tubular casting vessel 19.Operation of the levitator coil assembly in this manner assures optimumheat transfer between the liquid metal column 23 and the liquid cooledtubular casting vessel 19 while simultaneously reducing frictional,adhesive and gravitational forces acting on the liquid metal column to aminimum. From this description it will be appreciated that the tubularcasting vessel 19 serves not only as a casting vessel but also as a heatexchanger. Accordingly, hereafter, this component will be referred to astubular casting vessel/heat exchanger 19, 21.

The new and improved levitator assembly shown in FIG. 3 is comprised bya plurality of slotted arrays of annular slugs shown at 11A, 11B and 11Cwhich surround the portion of the length of the tubular castingvessel/heat exchanger 19, 21 within which the liquid metal column is tobe levitated while simultaneously being cooled. In the embodiment of theinvention shown in FIG. 3, each of the slotted annular slug arrays 11A,11B and 11C is comprised by a stacked array of slotted annular unitarymonolithic discs or slugs which are electrically insulated one from theother and are similar in construction to the slotted annular membershown in FIG. 1. The number and thickness of the slotted annular discs11 in each of the slug arrays 11A, 11B, 11C may vary in accordance withdesign criteria for a particular installation. Each monolithic slug isprovided with a thin electrical insulating coating which in the case ofaluminum slugs may comprise an anodized layer of aluminum oxide. Each ofthe slug arrays 11A, 11B, 11C thus comprised are also electricallyinsulated from each other by insulating members 10.

Each of the respective slug arrays 11A, 11B and 11C are inductivelycoupled to an associated multi-turn electromagnetic field producing coilsuch as 12A, 12B or 12C with the multi-turn coils being formed from alarge number of turns of insulated wire which may optionally be furtherinsulated from the exterior circumferential surfaces of the respectiveassociated slug member 11A, 11B or 11C by respective cylindricallyshaped insulated surfaces 10A, 10B or 10C in the manner shown in FIG. 3.In operation, each of the respective multi-turn windings 12A, 12B and12C is excited with a respective phase excitation current supplied froma multi-phase power source as will be described hereafter with relationto FIG. 4. The slug arrays 11A, 11B and 11C will operate in the mannerdescribed above with relation to FIG. 1 as a current step-up transformerfor converting the relatively high voltage, low current supplied to therespective phase windings 12A, 12B and 12C to a low voltage, highcurrent that flows around the periphery of central opening 14. This highcurrent produces a concentrated flux passing through the centralopenings of the respective slug array and acts on the liquid metalcolumn 23 contained in the tubular casting vessel/heat exchanger 19, 21.Due to the phasing of the excitation of the respective coil assemblies12A, 12B and 12C, an upwardly travelling electromagnetic wave isproduced which acts on liquid metal column 23 in the solidificationregion 24 so as to maintain the liquid metal column in this region in asubstantially weightless condition and which simultaneously provides acontainment field effect that maintains a minimal gap space between theexterior surfaces of the liquid metal column 23 and the interiorsurfaces of the tubular casting vessel/heat exchanger 19, 21.

Referring to FIG. 4, molten metal to be cast is contained in a holdingfurnace (not shown) from which it is delivered into a casting crucible31 as shown by the arrow 32 on an as required basis to maintain adesired level of liquid metal within the casting assembly 35 comprisedby the tubular casting vessel/heat exchanger portion 19, 21 and slottedannular slug member assembly 11A, 11B, 11C and surrounding multi-turncoils 12A, 12B and 12C described with relation to FIG. 3. The castingassembly 35 is mounted on and extends vertically upward from crucible 10to an open upper end through which the freshly cast solidified rodproduct 25 is withdrawn by means for removing solidified metal and forcontrolling the rate of production of solidified metal comprised by awithdrawal assembly for supplying the solidified metal to a precoolingstation 36 via an intermediate quenching station 36A. From theprecooling station 36 the freshly cast and precooled solidified rodproduct 25 may be delivered via withdrawal rolls 37 and 38 to tandemhot-rolling stations 39 and 41 (should such be required) and thenfinally cooled to ambient temperature and coiled at a coiling station 42for storage and delivery to a user of the cast product. Alternatively,the solidified rod product 25 can be withdrawn by withdrawal rolls 37and 43, cooled to ambient temperature and then stored without furtherprocessing. By controlling the rate of withdrawal of the solidifiedmetal product with withdrawal rolls 37, 38 or 37, 43, the rate ofproduction of solidified metal product is controlled.

In operation, molten metal is displaced from crucible 31 as a liquidmetal column such as shown at 23 in FIG. 3 into the casting assembly 35by gravity or pressurized flow from the holding furnace (not shown). Theholding furnace delivers the molten metal into crucible 31 at intervalsor continuously as necessary during the continuous casting process. Themolten metal column 23 (FIG. 3) is thus initially established andthereafter maintained at a level above that at which the upwardlytravelling levitation electromagnetic wave produced by the levitatorcoil assembly becomes effective to reduce or even eliminate the columnhydrostatic head. The upwardly travelling, levitation electromagneticwaves are produced in the manner described previously with respect toFIG. 3 as a result of multi-phase excitation currents supplied to therespective multi-turn inductor winding coils 12A, 12B and 12C from athree phase AC current supply and controller 26. Controller 26 iscontrolled independently in frequency and power by a respectivefrequency control circuit 27 and power control circuit 28 of knownconstruction.

While a three phase arrangement has been shown in FIG. 4 for simplicityof illustration, six phase excitation of the levitating coil assembly ispreferred. However, it is believed obvious to those skilled in the artthat other multi-phase power supply systems and coil arrangements couldbe employed. For example, as shown in FIG. 6, twelve multi-turn coils12A, 12(-B'), 12C, 12(-A'), 12B and 12(-C'), repeated a second time, aredisposed in vertical spaced relationship around the improved levitationcoil assembly 35 as windings arranged substantially normal to thecasting vessel/heat exchanger tube 19 axis. These coils are electricallyinterconnected to form a serially arranged, two-six phase coil systemthat physically extends over two wavelengths at the excitation frequencyof the coils to thereby determine the length of the levitation zone.Such an arrangement also is illustrated schematically in FIG. 5 of theabove referenced U.S. Pat. No. 4,414,285, the disclosure of which ishereby incorporated into this application in its entirety, but isdescribed as a twelve phase system. If it is desired to employ only asingle, six phase coil system extending over a single wavelength of theexcitation frequency of the coils, then the number of multi-turn coils12A, 12(-B'), etc., shown in FIG. 6 would be reduced to only a singleset of such coils and the electrical interconnections to the second setof coils eliminated. Other coil arrangments employing interconnectedgroups of three, four or other interconnected groups of phase windingcombinations will be obvious to those skilled in the art in the light ofthe above disclosure.

The improved multiphase levitator coil assembly described above producesa progressive upwardly travelling wave which will move at a speedproportional to the distance between successive closed flux loops andthe frequency of excitation. The primary multi-turn excitation windings12A, 12B and 12C are arrayed vertically upward along the length of thelevitator tube assembly 35 so that the liquid metal column and newlysolidified metal product in all but the lowermost section of levitatortube assembly 35 can be levitated throughout the casting operation to asubstantially weightless condition. In this condition the liquid metalcolumn 23 has substantially a zero hydrostatic head within thesolidification region of levitator tube 35 so that the liquid metalcolumn is substantially pressureless. By pressureless, it is meant thatthere is no substantial continuous pressure contact between the outersurface of the liquid metal column and the interior surrounding surfacesof the casting vessel 19 and the liquid metal column is withoutsubstantial hydrostatic head in the critical solidification zone 24. Asa result, frictional and adhesive forces as well as the force of gravityacting on the solidifying column are reduced to a minimum in thesolidification zone.

In order to limit the size of the casting equipment and particularly thelength of the levitator tube assembly 35 and also minimize the powerinput requirement to maintain the liquid metal column weightless throughthe solidification region, maximum heat exchange effectiveness isdesireable. The heat exchanger arrangement shown in FIG. 3 provides ineffect a condition approaching a water quench by effectively envelopingthe rising liquid metal column 23 in a continuous (during operation),rapidly flowing, turbulent but fairly small cross section annular streamof liquid coolant supplied via the upper manifold or header 33 anddrained through the lower header 34. The heat flow across the small gapbetween the liquid metal column 23 and surrounding graphite tube 19 thatbears against the cylindrical surface of the inner wall of annular heatexchanger 21, made from stainless steel or other similar material, ishighly effective. This heat transfer capability can be further enhancedby the inclusion of short, internal, annular ribs within annular coolingchamber 21 which serve as barriers to laminar flow of the liquidcoolant, causing turbulence in the cooling liquid as it travelsdownwardly through the annular heat exchanger from the upper manifold 33to the lower manifold 34.

The inside diameter of the tubular graphite casting vessel 19 shown inFIG. 3 and the operating parameters of the system such as the frequencyand field strength of the upwardly travelling levitating electromagneticfield are selected so that there is a minimum annular gap such asindicated at 22 between the exterior surfaces of the liquid metal column23 and the interior surfaces of tubular casting vessel 19 in thesolidification region defined by the interface 24. This is true belowthe point where solidification of the liquid metal column results inshrinkage of the column cross section area although such shrinkage isquite small. The gap indicated at 22 in FIG. 3 is schematic and notintended as an accurate representation of the location or the magnitudeof the dimensions of this annular gap. This gap, if allowed to becometoo large due to the containment effect produced by the upwardlytravelling levitating electromagnetic field in the solidification regionand just below it, could seriously impair effective heat transferbetween the liquid metal column 23 and the tubular casting vessel/heatexchanger 19, 21. This is due to the fact that there is a strong inverserelationship between field strength and heat removal rate. Consequently,the upwardly travelling, electromagnetic levitation field strengthshould be adjusted at the start of a casting operation so as to providepressureless contact as defined above with minimum gap spacing in thesolidification region consistent with good heat transfer in thiscritical region. Then the field strength should be maintained at thissetting and should not be changed during the course of the castingoperation even though the rate of movement (line speed) of the liquidmetal column through the levitator tube assembly and outgoing solidifiedmetal product might be changed.

From the standpoint of a practical continuous casting process, thetemperature of the solidified rod product is quite critical and must bemaintained within a relatively narrow range. For example, if the castrod product is copper and is much above 1,000 degrees Centigrade (whitehot) it will be too weak to support itself and transmit the tensileforces needed to move the rod from the casting operation in levitatortube assembly 35 through the optionally employed prequenching andprecooling chambers 36A, 36 via withdrawal rolls 37, 38. On the otherhand, if the rod temperature is less than about 850 degrees Centigrade,it will be too cold for the "hot" rolling which optionally may beprovided by tandem rolls 39, 41 if this is desired to create a finegrain, homogenous structure which is optimum for subsequent cold drawing(or cold working) of the solidified metal. There is a considerableadvantage from the standpoint of overall system cost and processsimplicity to eliminate the hot rolling apparatus, if possible.Fortunately and unexpectedly, the intense agitation and stirring actionof the electromagnetic levitation field results in cast rod having amoderate size grain structure that appears to be useable "as-is". Forsuch applications it is adequate to just use a spray or mist-type cooler36, 36A above the levitation coil/heat exchanger assembly 35 to "quench"the emerging solidified rod, and then feed the rod directly into acoiler or other take-up mechanism via withdrawal rolls 37 and 43.

Due to the above considerations, the recommended procedure is that thecasting speed (i.e. line speed of movement of the liquid metal columnthrough the levitator tube assembly 35) should be controlled bycontrolling the drive motors for the rod withdrawal rolls 37, 38 or 37,43 as shown in FIG. 4. The levitation field strength and excitationfrequency should be established at a value calculated for the particularsize and resistivity of the metal being cast to give a levitation ratioin the range between 75% to 200% where levitation ratio is defined asthe ratio of the levitation force per unit of length of the liquid metalto the weight per unit length of the liquid metal as expressed in U.S.Pat. No. 4,414,285 at the bottom of column 11 and the top of column 12.The excitation frequency is determined by the expression F=36ρ/D² asdescribed and explained more fully in U.S. Pat. No. 4,414,285 where F isthe frequency in kilohertz, ρ is the resistivity in micro-ohm-cm, and Dis the average rod diameter in millimeters. Thereafter, during thecourse of the run, both the excitation frequency chosen and theelectromagnetic levitation field strength should be maintained and notchanged during the run.

As noted above in the description of FIG. 3, the multi-turn coils 12A,12B and 12C are fabricated from ordinary high temperature insulatedwire. Since the wires would be carrying relatively modest currents, itis likely that they would not have to be cooled separately with theirown liquid cooled heat exchanger arrangement although the provision offorced cooling air flow over the coils may be necessary. The slottedannular slug members 11 of course are in close proximity to the annularheat exchanger water jacket 21 and have a relatively large conductingcross section so that they can be maintained at a modest temperaturedespite the high currents flowing through them. The thickness of theslotted annular discs comprising annular slugs 11A, 11B and 11C can beadjusted over a wide range to enhance this capability. It isanticipated, however, that at least two to three discs per slottedannular slug multi-turn coil arrangement for each phase would beemployed in order to create a more uniform field around the centralopening in which the tubular casting vessel/heat exchanger 19, 21 isdisposed.

The slots 15 formed in the discs comprising the slotted annular slugs donot have to be lined-up vertically, since the discs comprising the slugare insulated one from the other, but instead could be oriented from onedisc to another in such a way that the overall field distortion (if any)resulting from the slots can be minimized. Further, it is believedapparent to those skilled in the art that the central opening within theslotted annular slug members 11A, 11B, 11C, etc., can be of any desiredshape in cross section. For example, either oval, hexagonal or otherdesired cross sectional configuration could be used to cast solidifiedrod product of a similar cross section. In a similar manner, the outerperiphery of the slotted annular slugs do not have to be circular inshape and could be oval, hexagonal, or other desired configurations.

The outer surfaces of the flux concentrator slotted annular slugs do nothave to be smooth but in an effort to get better coupling to theirrespective multi-turn energizing windings, they could be provided withgrooves cut annularly around the outer surface. If a single continuousmulti-turn winding is to be used, a continuous spiral groove could beemployed. Additionally, in accordance with good engineering practice, itmay be desireable to make the multi-turn energizing coils 12A, 12B, 12Cfrom one or a few layers of square or rectangularly cross sectionconductors instead of several layers of wound round conductors. Thistype of construction would minimize the effective air gap between theenergizing coil and its associated flux concentrator slotted annularslug. Also, it could provide other benefits at higher energizingfrequencys such as reduced capacitance. Energizing coils formed fromrectangular cross section conductors, because of their larger effectiveconducting cross sections, also would have lower I² R heating lossesthan coils made from multiple turns of insulated round wires. The choiceof insulation used in fabricating the multi-turn energizing coilconductors also is a matter of good engineering practice. For example,use of a high temperature wire enamel, polymeric coatings or tape, andother similar newly developed high temperature insulating materialsconceivably could eliminate the need for or reduce the cost andcomplexity of cooling arrangements for the coil assemblies.

Other modifications and variations in the construction to the fluxconcentrator assembly will be suggested to those skilled in the art inthe light of the above teachings. For example, it is possible toincorporate ferromagnetic material such as specially-shaped hightemperature ferrite members in the construction of the primarymulti-turn energizing coil slotted annular slug flux concentratorassembly. It is suspected that the electric field below the bottom coilin the levitator assembly, because of its distance from the interactingfields produced by the other coils, may act essentially like a singlephase field that tends to repel the upward movement of the liquid metalcolumn. By suitably fashioning and placing ferromagnetic ferrite membersat the bottom of the stacked assembly, it may be possible to minimizethe effect of this repulsion field. Further, if a central opening in theslotted annular slug member flux concentrator plate is employed havingother than a circular cross section, suitably configured ferromagneticferrite material flux shaping members could be incorporated into theassembly along with the slotted annular slug members in order to"shape-up" the electromagnetic field produced by the flux concentratorassembly into a desired field pattern. Such an arrangement will bedescribed hereafter with relation to FIG. 7. Another method of fieldshaping might be to cut out segments or additional field shaping slotsaround either the inner or outer peripheries of the slotted annular slugmembers whereby currents flowing in an undesired manner are forced intocurrent paths providing a more optimum magnetic field configuration.Removing segments or additional slots of the slotted annular slug memberfor field shaping purposes can be achieved but at the expense of lostfield and an increase the electrical impedance of the plates. Hence,this method of field shaping might be less desireable than thatemploying ferromagnetic ferrite field shaping members even thoughferromagnetic components are known to be non-linear in high frequencyfields.

From the standpoint of providing a practical operating facility of goodengineering design, it would be most desireable to provide the new andimproved levitation coil assembly with a design such that one set ofprimary multi-turn energizing coils having a fixed cross sectionalinternal opening could be used with a variety of slotted annular slugflux concentrator-heat exchanger assemblies having various differentcentral opening diameters but a constant outer cross sectionalconfiguration and area. A user of the GELEC process incorporating thenew and improved levitation coil assembly constructed in this mannerthen could change over from making 8 mm diameter rod, for example, to 5mm diameter rod by only changing the internal slotted annular slug fluxconcentrator-heat exchanger assembly and not have to remove or alter theprimary, multi-turn, outer energizing coils themselves.

In a preceeding paragraph it was indicated that the cross sectionalopening provided in the center of the flux concentrator slug memberscould be other than circular in cross section. FIGS. 7, 7A and 7B of thedrawings illustrate one such arrangement. FIG. 7 is a top planar view ofonly one phase winding of an installation suitable for use infabricating plates from molten metal having a rectangular cross sectionas shown at 59 in FIGS. 7 and 7A. The rectangular cross section moltenmetal plate 59 is formed by reason of the generally rectangular fluxconcentrator slug member 55 having an elongated rectangular centralopening 61 and a gap formed therein as shown at 58 in both FIGS. 7 and7B of the drawings. The rectangular-shaped flux concentrator 55 isdisposed within an outer primary multi-turn coil 56 best seen in FIGS. 7and 7B. In order to better shape the magnetic field flux emanating fromthe flux concentrator slug member 55, a plurality of nonconducting, thinferrite plates shown at 57 are disposed over and under the multi-turnprimary coil 58 and flux concentrator slug member 55 subassembly as bestshown in FIG. 7A. The thin ferrite plate members 57 havespecially-shaped trapezoidal configurations as best seen in FIG. 7 forconcentrating the magnetic flux in the longer dimension flat section ofthe molten metal plate 59 whereby the plate is provided with a generallyflat rectangular cross section as illustrated in FIG. 7. Similar to theembodiment of the invention shown in FIGS. 3 and 4, a graphite lined,water cooled heat exchanger that comprises the casting vessel/heatexchanger 19, 21 is positioned between the flux concentrator slottedannular slug assembly 11A, 11B and 11C and the liquid metal column 23being levitated. It will be appreciated therefor that relatively heavycurrents will be induced in the liquid cooled heat exchanger in thisconstruction and will result in rather substantial losses. In order toavoid such losses, a preferred embodiment of the invention is providedwhich is illustrated in FIGS. 5 and 6 of the drawings.

From a consideration of FIG. 6, it will be appreciated that the slottedannular slug flux concentrator plate asssembly shown generally at 11 inFIG. 6 is mechanically strong and rigid. If the flux concentrator slugs11 are made of a metal having good heat conductivity but also capable ofproviding electrical isolation between the respective slug members, suchan assembly would also be capable of transfering a considerable amountof heat to a liquid coolant flowing from the upper and lowerelectrically insulating header manifolds 33 and 34 down through a seriesof cooling apertures shown at 51 in FIG. 5 formed in each of the slugmembers 11. In this arrangement the tubular casting vessel 19 couldcomprise a refractory lining vessel such as graphite, zirconia, TZM andthe like which is press fit directly into the inner opening 14 of thestacked array of slotted annular slugs 11A, 11(-B'), 11C, etc., thatform a liquid cooled slotted annular slug flux concentrator assembly 35.Assembly 35 functions in the same manner as the flux concentratorassembly 35 described with relation to FIG. 4. However, in the FIGS. 5and 6 assembly, the reduced spacing between the inner periphery of theslotted annular slug flux concentrator assembly 35 and the levitatedliquid metal column 23 contained within the tubular casting vesselportion 19, greatly improves the electromagnetic coupling to liquidmetal column 23 and eliminates the power lost by eddy currents inducedin the heat exchanger assembly 21 of the FIG. 3 assembly. Further, byelimination of the annular heat exchanger member 21 of the FIG. 3assembly, the cost of the overall assembly is reduced.

It should be further noted with respect to FIG. 6 that each of theslotted annular slug flux concentrators 11A, 11(-B'), 11C, etc.,comprises a relatively thick monolithic slug whose axial dimensions aresubstantially equal to the axial dimension of its associated primarymulti-turn driving coil such as 12A, 12(-B'), 12C, etc. These multi-turncoils are respectively excited from a three phase current supply andcontroller 26 and are connected thereto in the manner described morefully above and with respect to FIG. 5 of the drawings of U.S. Pat. No.4,414,285 referenced above.

The improved levitator coil and heat exchanger assembly should bedesigned so that the slotted annular slug members are press fit withinthe surrounding associated primary multi-turn driving coil and still areelectrically insulated from their associated primary multi-turn coil,the tubular refractory liner 19 and from adjacent slug members. For thispurpose, it is anticipated that the slotted annular slug members 11employed in the FIG. 6 arrangement would be fabricated from a softaluminum material such as aluminum 1100. The cooling passageway 51 couldbe drilled or cast therein and each of the annular slug members thenanodized by known electrochemical means. The anodizing treatment willresult in the production of an aluminum oxide film being grown aroundall of the exposed surfaces of the slug to a thickness of about 2/1000of an inch. The aluminum oxide film thus provided will electricallyinsulate each of the slotted annular slug members one from the other aswell as from its associated primary multi-turn driving coil and thetubular refractory liner 19. The stacked array of slotted annular slugmembers 11A, 11(-B'), 11C, etc., is pressed together with coolingpassageways 51 aligned and forming liquid tight seals between therespective slug members. Since the interior surfaces of the coolingpassageways 51 likewise will have an aluminum oxide insulating surfacegrown therein, the liquid coolant will be unable to electrically shortout between adjacent slug members. If desired, copper, aluminum or othertubes could be inserted into the aligned openings 51 and then expandedto provide a press fit, thereby further insuring that no leakage occursbetween adjacent slug members. The anodized coating of aluminum oxideprevents the copper or other conductive tubes from shorting between theslotted annular slug members.

From the above description it will be seen that the individual slottedannular slug flux concentrator members will be electrically insulatedone from the other by the anodized aluminum coating so that the largeflux producing currents induced around the periphery of the inneropenings 14 thereof can be individually controlled to produce thedesired upwardly travelling electromagnetic levitation field required topractice the GELEC (TM) process. Further, it is known that the thermalresistivity of thin aluminum oxide anodized coating is minimal becauseof its thinness. Thus, the cooling characteristics of the assembledlevitating coil structure are comparable to or perhaps even better thatthe cooling provided with the assembly shown in FIG. 3. If desired,other aluminum materials such as aluminum 2024 could be employed informing the slugs although such material is known to have a somewhathigher resistivity than aluminum 1100 and would lead to somewhat higherlosses during operation of the levitator coil assembly.

If it is determined that an improved levitation field producing assemblysuch as that shown in FIGS. 5 and 6 does not provide adequate coolingfor certain products, it is possible to provide additional cooling tothat provided by passageways 51. For that matter, it may be necessary toprovide such additional fluid cooling passageways in the slotted annularslug members of the arrangement shown in FIG. 3, although one of theexpected benefits of the use of the flux concentrator slotted annularslug members is to eliminate the need for water cooled conductors in thelevitation coil assembly. However, it may prove necessary for the fluxconcentrator slotted annular slug member assembly of FIG. 3 to be cooledeither by air or water or some other liquid coolant. If such cooling isneeded, cooling channels such as those shown in FIG. 5 could beincorporated into the slotted annular slug members of FIG. 3. Suchcooling channels should be positioned several electrical skin depthsaway from the inner and outer peripheries of the respective slug membersso as not to impede or distort the flow of the levitation fieldproducing currents.

The above consideration may ultimately limit the number of coolingpassageways, such as 51, that can be formed in the respective slottedannular flux concentrating slug members. Should that prove to be true,and additional cooling still be required to practice the GELEC (TM)process, then it is possible that the slug members could be hollowed outand provided with annular cooling passageways of the type described withrelation to FIGS. 9-16 of U.S. Pat. No. 3,872,379, for example. Theannular fluid passageways thus formed could be interconnected betweenthe stacked assemblage of slotted annular slug members viainterconnecting apertures such as shown at 51 in FIG. 5. It may bepossible to eliminate the need for the casting vessel 19 liner bysuitable design and use of anodized flux concentrator slug members 11.Other modifications and variations required to produce the desiredamount of cooling will be suggested to those skilled in the art in thelight of this disclosure.

INDUSTRIAL APPLICABILITY

This invention describes an electromagnetic levitation casting apparatushaving an improved levitator coil assembly for use in the continuouscasting of metal products of long length such as rod made from copper,aluminum, nickle and various alloys of these and other metals.

Having described several embodiments of an electromagnetic levitationcasting apparatus having improved levitation coil assembly constructedin accordance with the invention, it is believed obvious that othermodifications and variations of the invention will be suggested to thoseskilled in the art in the light of the above teachings. It is thereforeto be understood that changes may be made in the particular embodimentsof the invention described which are within the full intended scope ofthe invention as defined by the appended claims.

What is claimed is:
 1. Continuous casting apparatus comprising anelongated tubular casting vessel disposed in upright position to receiveliquid metal for solidifiction, means for delivering liquid metal into alower portion of the vessel, heat exchange means associated with thevessel for cooling and solidifying liquid metal therein, means forremoving solidified metal from an upper portion of the vessel, andelectromagnetic levitation field producing means disposed around thevessel along a portion of its length for reducing the hydrostatic headof the column and maintaining a predetermined dimensional relationshipbetween the outer surface of the liquid metal column and the interiorsurrounding surfaces of the casting vessel to thereby effect maximumobtainable heat transfer between the liquid metal column and the castingvessel while simultaneously reducing gravitational, frictional andadhesive forces of the liquid metal column to a minimum, saidelectromagnetic levitation field producing means including magneticfield concentrating means comprised by a compact, closely stacked arrayof slotted annular slugs surrounding the portion of the length of thetubular casting vessel within which the liquid metal column is to belevitated, each of the slugs being insulating supported within thecompact array, one from the other by a thin insulating coating andinductively coupled to a respective electromagnetic field producing coilhaving a large number of turns surrounding the slug whereby each slugserves to concentrate the magnetic field produced by the coil tosubstantially the interior cross sectional area of the portion of thetubular casting vessel it surrounds and functions as a current step-uptransformer.
 2. The apparatus of claim 1 in which the electromagneticlevitation field producing means includes a plurality of electromagneticcoils for connection to successive phases of a polyphase alternatingelectric current source for producing an upwardly travelling alternatingelectromagnetic field and at least one slotted annular slug andrespective electromagnetic field producing coil magnetic fieldconcentrating means is provided for each of the successive phases withthe slotted annular slugs for each of the phases being electricallyinsulated one from the other.
 3. The apparatus of claim 1 wherein theslotted annular slug inductively coupled to the electromagnetic fieldproducing coil for each phase comprises a monolithic structure having ananodized insulating surface formed thereon.
 4. The apparatus of claim 2wherein the slotted annular slugs are arranged within the compactstacked array in a manner such that the open slots of the respectiveslotted annular slugs are not vertically aligned with respect to eachother and a uniform balanced electromagnetic field is produced by thecompact stacked array of the slotted annular slugs.
 5. The apparatus ofclaim 1 wherein the heat exchange means comprises an annularly-shapedfluid cooled heat exchanger immediately surrounding the tubular castingvessel in the region thereof where said electromagnetic levitation fieldproducing means is disposed, said tubular casting vessel andannularly-shaped fluid cooled heat exchanger being positioned in thecentral opening of the slotted annular slugs comprising the magneticflux concentrators for the electromagnetic field producing means, andmeans for continuously supplying cooling fluid to the annularly-shapedfluid cooled heat exchanger.
 6. The apparatus of claim 1 wherein theheat exchange means is comprised in part by the slotted annular slugswhich immediately surround and are in mechanical and thermallyconductive contact with the outer surfaces of the tubular casting vesselin the region thereof where said electromagnetic levitation fieldproducing means is disposed but are electrically insulated therefrom,said slotted annular slugs having passageways formed therein for thepassage of cooling fluid, and means for continuously supplying coolingfluid to the cooling passageways formed in said slotted annular slugs.7. The apparatus of claim 2 in which the tubular casting vessel is atube of refractory material of substantially uniform inside diameter andfurther includes a crucible to contain molten metal communicating withthe lower end of the tubular casting vessel, means associated with thecrucible to establish and move a column of liquid metal upwardly intothe tubular casting vessel to a level above the lower end of theelectromagnetic levitation field producing means, means for joining thelower end of a starting metal rod to the upper end of the molten liquidmetal column within the electromagnetic levitation field, means formaintaining the value of the electromagnetic field so that the crosssectional dimension of the liquid metal column is sufficiently large topreclude formation of a substantial gap between the outer surface of thecolumn and the interior surrounding surfaces of the casting vessel,means independent from said electromagnetic levitation field producingmeans for moving the liquid metal column upwardly through the castingvessel, and means for controlling the rate of production of solidifiedmetal product by controlling the rate of removal of the solidified metalproduct from the upper portion of the tubular casting vessel.
 8. Theapparatus of claim 7 in which the polyphase alternating electric currentsource is a multi-phase generator whose output power and frequency canbe variably controlled to produce a uniform and balanced upwardlytravelling electromagnetic levitating force in accordance with the typeand size of metal being cast.
 9. The apparatus of claim 8 wherein theslotted annular slug inductively coupled to the electromagnetic fieldproducing coil for each phase comprises a monolithic structure having ananodized insulating surface formed thereon.
 10. The apparatus of claim 8wherein the heat exchange means comprises an annularly-shaped fluidcooled heat exchanger immediately surrounding the tubular casting vesselin the region thereof where said electromagnetic levitation fieldproducing means is disposed, said tubular casting vessel andannularly-shaped fluid cooled heat exchanger being positioned in thecentral opening of the slotted annular slugs comprising the magneticflux concentrators for the electromagnetic field producing means, andmeans for continuously supplying cooling fluid to the annularly-shapedfluid cooled heat exchanger.
 11. The apparatus of claim 8 wherein theheat exchange means is comprised in part by the slotted annular slugswhich immediately surround and are in mechanical and thermallyconductive contact with the outer surfaces of the tubular casting vesselin the region thereof where said electromagnetic levitation fieldproducing means is disposed but are electrically insulated therefrom,said slotted annular slugs having passageways formed therein for thepassage of cooling fluid, and means for continuously supplying coolingfluid to the cooling passageways formed in said slotted annular slugs.12. The apparatus of claim 8 wherein the slotted annular slugs arearranged within the compact stacked array in a manner such that the openslots of the respective slotted annular slugs are not vertically alignedwith respect to each other and a uniform balanced electromagnetic fieldis produced by the compact stacked array of slotted annular slugs. 13.The apparatus of claim 12 wherein the heat exchange means comprises anannularly-shaped fluid cooled heat exchanger immediately surrounding thetubular casting vessel in the region thereof where said electromagneticlevitation field producing means is disposed, said tubular castingvessel and annularly-shaped fluid cooled heat exchanger being positionedin the central opening of the slotted annular slugs comprising themagnetic flux concentrators for the electromagnetic field producingmeans, and means for continuously supplying cooling fluid to theannularly-shaped fluid cooled heat exchanger.
 14. The apparatus of claim12 wherein the heat exchange means is comprised in part by the slottedannular slugs which immediately surround and are in mechanical andthermally conductive contact with the outer surfaces of the tubularcasting vessel in the region thereof where said electromagneticlevitation field producing means is disposed but are electricallyinsulated therefrom, said slotted annular slugs having passagewaysformed therein for the passage of cooling fluid, and means forcontinuously supplying cooling fluid to the cooling passageways formedin said slotted annular slugs.
 15. The apparatus of claim 12 furtherincluding means for precooling the solidified metal product as itemerges from the upper portion of the casting vessel, means for rollingthe product to a desired dimension and means for cooling the rolledproduct to an ambient temperature for storage and subsequent use. 16.The apparatus of claim 12 further including means for precooling thesolidified metal product and thereafter cooling the precooled solidifiedproduct to an ambient temperature for storage and subsequent use.
 17. Ina continuous casting apparatus including an elongated tubular castingvolume disposed in an upright position to receive liquid metal forsolidification, means for delivering liquid metal into a lower portionof the casting volume, heat exchange means associated with the castingvolume for cooling and solidifying liquid metal therein, means forremoving solidified metal product from an upper portion of the castingvolume, and electromagnetic levitation field producing means disposedaround the casting volume along a portion of its length for producing anelectromagnetic levitation field that reduces the hydrostatic head ofthe column and maintains the liquid metal column in a substantiallyweightless condition while simultaneously maintaining a predetermineddimensional relationship between the outer surface of the liquid metalcolumn and the interior surrounding surfaces of the casting volume tothereby assure optimum heat transfer between the liquid metal column andthe casting volume while simultaneously reducing frictional, adhesiveand gravitational forces acting on the column to a minimum; theimprovement wherein the electromagnetic levitation field producing meansincludes magnetic field concentrating means comprised by a compact,closely stacked array of slotted annular slugs surrounding the portionof the length of the tubular casting volume within which the liquidmetal column is to be levitated, each of the slugs being insulatingsupported within the compact array one from the other by a thininsulating coating and inductively coupled to a respectiveelectromagnetic field producing coil having a large number of turnssurrounding the slug whereby each slug serves to concentrate themagnetic field produced by its coil to substantially the interior crosssectional area of the portion of the tubular casting volume it surroundsand functions as a current step-up transformer.
 18. The apparatus ofclaim 17 in which the electromagnetic levitation field producing meansincludes a plurality of electromagnetic coils for connection tosuccessive phases of a polyphase alternating electric current source forproducing an upwardly travelling alternating electromagnetic field andat least one slotted annular slug and respective electromagnetic fieldproducing coil magnetic field concentrating means is provided for eachof the successive phases with the slotted annular slugs for each of thephases being electrically insulated one from the other.
 19. Theapparatus of claim 18 wherein the slotted annular slug inductivelycoupled to the electromagnetic field producing coil for each phasecomprises a monolithic structure having an anodized insulating surfaceformed thereon.
 20. The apparatus of claim 18 wherein the slottedannular slugs are arranged within the compact stacked array in a mannersuch that the open slots of the respective slotted annular slugs are notvertically aligned with respect to each other and a uniform balancedelectromagnetic field is produced by the compact stacked array ofslotted annular slugs.
 21. The apparatus of claim 20 wherein the heatexchange means comprises an annularly-shaped fluid cooled heat exchangerimmediately surrounding the tubular casting volume in the region thereofwhere said electromagnetic levitation field producing means is disposed,said tubular casting volume and annularly-shaped fluid cooled heatexchanger being positioned in the central opening of the slotted annularslugs comprising the magnetic flux concentrators for the electromagneticfield producing means, and means for continuously supplying coolingfluid to the annularly-shaped fluid cooled heat exchanger.
 22. Theapparatus of claim 20 wherein the heat exchange means is comprised inpart by the slotted annular slugs which immediately surround and are inthermally conductive relationship with the outer regions of the tubularcasting volume in the region thereof where said electromagneticlevitation field producing means is disposed but are electricallyinsulated therefrom, said slotted annuular slugs having passagewaysformed therein for the passage of cooling fluid, and means forcontinuously supplying cooling fluid to the cooling passageways formedin said slotted annular slugs.
 23. The apparatus of claim 20 furtherincluding means for establishing the strength of the electromagneticfield to provide a levitation ratio between 75% and 200% of the weightper unit length of liquid metal and means for adjusting the frequencyvalue of the frequency of excitation of the electromagnetic fieldproducing means over a range of values including an optimum frequencyvalue F=36ρ/D² where F is the frequency in kilohertz, ρ is theresistivity in micro-ohm-cm of the liquid metal column and D is theaverage diameter in millimeters of a solidified metal product producedby the apparatus, a crucible to contain a bath of molten liquid metalcommunicating with the lower end of the tubular casting volume, meansassociated with the crucible to establish and move a column of liquidmetal upwardly into the lower end of the tubular casting volume to alevel above the lower end of the electromagnetic levitation fieldproducing means, means for joining the lower end of a starting metal rodto the upper end of the molten liquid metal column within theelectromagnetic field of the electromagnetic levitation field producingmeans, control means for controlling the magnitude of the current andhence the magnetic field strength produced by the electromagnetic fieldproducing means to thereby control the levitation ratio produced by theapparatus, and means for controlling the rate of production ofsolidified metal product by controlling the rate of removal of thesolidified metal product from the upper portion of the casting volume.24. The apparatus of claim 23 wherein the slotted annular sluginductively coupled to the electromagnetic field producing coil for eachphase comprises a monolithic structure having an anodized insulatingsurface formed thereon.
 25. The apparatus of claim 23 wherein the heatexchange means comprises an annularly-shaped fluid cooled heat exchangerimmediately surrounding the tubular casting volume in the region thereofwhere said electromagnetic levitation field producing means is disposed,said tubular casting volume and annularly-shaped fluid cooled heatexchanger being positioned in the central opening of the slotted annularslugs comprising the magnetic flux concentrators for the electromagneticfield producing means, and means for continuously supplying coolingfluid to the annularly-shaped fluid cooled heat exchanger.
 26. Theapparatus of claim 23 wherein the heat exchange means is comprised inpart by the slotted annular slugs which immediately surround and are inthermally conductive relationship with the outer surfaces of the tubularcasting volume in the region thereof where said electromagneticlevitation field producing means is disposed but are electricallyinsulated therefrom, said slotted annuular slugs having passagewaysformed therein for the passage of cooling fluid, and means forcontinuously supplying cooling fluid to the cooling passageways formedin said slotted annular slugs.
 27. The apparatus of claim 23 wherein theslotted annular slugs are arranged within the compact stacked array in amanner such that the open slots of the respective slotted annular slugsare not vertically aligned with respect to each other to provide auniform and balanced electromagnetic field by the stacked array ofslotted annular slugs.
 28. The apparatus of claim 27 wherein theelectromagnetic field producing coil for each phase has a standard sizeand shape central opening into which a plurality of differently designedslotted annular slug members having a standard size and shape externalconfiguration can fit but which have different size and/or shapedcentral openings which define the space for accomodating the tubularcasting volume.
 29. The apparatus of claim 20 wherein theelectromagnetic field producing coil for each phase has a standard sizeand shape central opening into which a plurality of differently designedslotted annular slug members having a standard size and shape externalconfiguration can fit but which have different size and/or shapedcentral openings which define the space for accomodating the tubularcasting volume.
 30. The apparatus of claim 28 further including meansfor precooling the solidified metal product as it emerges from the upperportion of the casting vessel, means for rolling the product to adesired dimension and means for cooling the rolled product to an ambienttemperature for storage and use.
 31. The apparatus of claim 28 furtherincluding means for precooling the solidified metal product andthereafter cooling the precooled solidified product to an ambienttemperature for storage and use.
 32. The apparatus of claim 20 furtherincluding means for precooling the solidified metal product as itemerges from the upper portion of the casting vessel, means for rollingthe product to a desired dimension and means for cooling the rolledproduct to an ambient temperature for storage and use.
 33. The apparatusof claim 20 further including means for precooling the solidified metalproduct and thereafer cooling the precooled solidified product to anambient temperature for storage and use.